<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>多类名选择器</title>
    <style>
        .font {
            width: 100px;
            height: 100px;
        }
        .c {
            background-color: #000;
        }
        .b {
            background-color: green;
        }
        .a {
            background-color: red;
        }
       .d {
           color: #fff;
       }
        .c span{
            color: #fff;
        }
    </style>
</head>
<body>
    <div class="a  b font c">
        <span>123</span>4
    </div>
    <div class="b font"></div>
    <div class="a font"></div>
</body>
</html>